1. Preheat the oven to 450F.
2. Wash, scrub and dry the sweet potatoes.
3. Place a drying rack on top of a cookie sheet and put the sweet potatoes on top. By baking on a rack, no part of the skin will get soggy which will make stuffing them easier. Bake for approximately an hour turning every 20 minutes. (The sweet potatoes are done when they are soft to the touch and just starting to puff up, so you can tell the flesh is shifting away from the skin.)
4. Remove from the oven and let cool enough to handle, about 15 minutes.
5. Using a sharp knife gently slice away the top third of the sweet potato. With a small spoon carefully scoop away the inside of the sweet potato and put the flesh into a medium bowl.
6. Mash the flesh with a fork and whip in cumin, coriander, salt and a generous amount of freshly ground pepper. Stir in tomatoes, scallions, black beans and lime juice.
7. Divide the mixture in half and carefully stuff back into the potato skins, you can overfill them as long as they can stay upright.
8. Place potatoes back on the rack and bake for another 15 minutes.
9. Serve hot garnished with cilantro.